1. Field of the Invention
The present invention relates to a coil load driving circuit that drives a coil load in the positive or negative direction via pulse width modulation (PWM) and also relates to an optical disc device that performs focus adjustment or tracking adjustment using such a coil load driving circuit.
2. Description of the Related Art
As an example of such a coil load driving circuit, there is known (see, for example, Japanese Patent Application Laid-open No. 2003-164194) a coil load driving circuit that controls the torque of a motor by a PWM signal and that controls the forward or reverse direction of rotation of the motor, constituting a coil load, by a polarity signal. An example of such a coil load driving circuit is shown in FIG. 7. This coil load driving circuit 101 inputs an input control voltage VIN through an input terminal IN from outside and inputs an input reference voltage VREF through an input terminal REF, and drives the motor 5 in the forward or reverse direction of rotation by applying a PWM pulse, responsive to the difference between the inputted two voltages, to the either terminal of the motor 5 through a first output terminal OUT1 or second output terminal OUT2. If the first output terminal OUT1 is of positive voltage with respect to the second output terminal OUT2, the motor 5 rotates in the forward direction and if the first output terminal OUT1 is a negative voltage with respect to the second output terminal OUT2, the motor 5 rotates in the reverse direction.
This coil load driving circuit 101 includes a voltage-current converter 131 that outputs a current proportional to the absolute value of the difference of an input control voltage VIN and input reference voltage VREF; a bias resistor 132 having one end that is connected with the output of the voltage-current converter 131 and another end that is connected with an adjustment voltage VADJ of predetermined voltage; a PWM comparator 114 that inputs at its inversion input terminal a triangular wave signal TRI that is output by an oscillator (OSC) 113 and inputs at its non-inversion input terminal a voltage (transfer voltage VTR) that is generated by passage of the output current of the voltage-current converter 131 to a bias resistor 132, and that outputs a PWM signal PW as a result of comparing these signals; a polarity comparator 115 that inputs at its inversion input terminal the input reference voltage VREF and inputs at its non-inversion input terminal the input control voltage VIN, and that outputs the result of the magnitude comparison of these voltages, i.e. a polarity signal PO indicating the polarity of the input control voltage VIN with respect to the input reference voltage VREF; a switch 116 that changes over the output of the PWM signal PW between two paths in accordance with the polarity signal PO; and first and second output buffers 111, 112 that output a PWM pulse to either terminal of the motor 5, being respectively connected to the two paths of the PWM signal PW. The above-described adjustment voltage VADJ is set to a lower value than the lower end voltage TRILOW of the triangular wave signal TRI. The switch 116 changes over the PWM signal PW to output to the second output buffer 112 when the polarity signal PO is low-level and changes over the PWM signal PW to output to the first output buffer 111 when the polarity signal PO is high-level, and outputs ground potential on the side where no PWM signal PW is output.
The operation of this coil load driving circuit 101 will now be described with reference to the waveform diagram of FIG. 8. FIG. 8 shows the waveforms at the various sections when the input control voltage VIN rises linearly. Referring to FIG. 8, (a) shows the input control voltage VIN, (b) shows the transfer voltage VTR and the triangular wave signal TRI, (c) shows the PWM signal PW, (d) shows the polarity signal PO, (e) shows the PWM pulse of the first output terminal OUT1 and (f) shows the PWM pulse of the second output terminal OUT2. As is clear from this waveform diagram, the transfer voltage VTR IS higher, and the pulse width of the PWM signal PW (i.e., the high-level period) is larger, the higher the difference between the input control voltage VIN and input reference voltage VREF. When the input control voltage VIN is lower than the input reference voltage VREF, when the input control voltage VIN rises, the transfer voltage VTR falls, and the pulse width of the PWM signal PW gradually becomes smaller. This PWM signal PW, since the polarity signal PO is low-level, is output as a PWM pulse from the second output buffer 112. At this point, the first output buffer 111 is fixed at ground potential. If the input control voltage VIN is higher than the input reference voltage VREF, when the input control voltage VIN rises, the transfer voltage VTR rises and the pulse width of the PWM signal PW gradually becomes larger. This PWM signal PW, since the polarity signal PO is high-level, is output as a PWM pulse from the first output buffer 111. At this point, the second output buffer 112 is fixed at ground potential.
In this way, with this coil load driving circuit 101, the torque that drives the motor 5 is controlled by outputting a PWM pulse of a pulse width responsive to the difference between the input control voltage VIN and the input reference voltage VREF from the first output buffer 111 or second output buffer 112. Also, the control to effect forward or reverse rotation of the motor 5 is performed by changing over either of the first output buffer 111 or second output buffer 112 to output the PWM pulse, in accordance with the polarity of the input control voltage VIN with respect to the input reference voltage VREF.
Since, in this case, as described above, the adjustment voltage VADJ is set to be lower than the lower end voltage TRILOW of the triangular wave signal TRI, an insensitive zone is produced between the lower end voltage TRILOW of the triangular wave signal TRI and the minimum voltage of the transfer voltage VTR. In this insensitive zone, while the absolute value of the difference between the input control voltage VIN and the input reference voltage VREF is no more than a predetermined value, no PWM pulse is output from the first and second output buffers 111, 112. FIG. 9(a) is a characteristic diagram showing the relationship of the transfer voltage VTR and upper end voltage TRIHIGH and lower end voltage TRILOW of the triangular wave signal TRI with respect to the difference between the input control voltage VIN and the input reference voltage VREF (INPUT on the horizontal axis). FIG. 9(b) is a characteristic diagram of the input/output showing the relationship of the DC voltage (average voltage) (OUTPUT on the vertical axis) between the two terminals of the motor 5 with respect to the difference of the input control voltage VIN and the input reference voltage VREF (INPUT on the horizontal axis). As shown in FIG. 9(b), the coil load driving circuit 101 maintains monotonicity (monotonousness) of the input/output characteristic by providing this insensitive zone. If no insensitive zone were to be provided, i.e. if the lower end voltage TRILOW of the triangular wave signal TRI and the minimum voltage of the transfer voltage VTR were to coincide, a characteristic diagram as shown in FIGS. 10(a) and 10(b) would be produced. Typically, an amplifier, comparator or voltage-current converter etc that is arranged to deliver output by comparison of two input voltages has some degree of input offset voltage. If there is a relative deviation of the input offset voltage of the voltage-current converter 131 and polarity comparator 115 in the coil load driving circuit 101, as shown in FIG. 10(b), in the vicinity of zero difference between the input control voltage VIN and input reference voltage VREF, spurious inversion of the polarity comparator 115 may take place, causing monotonicity of the input/output characteristic to be lost. An insensitive zone is therefore provided by setting the adjustment voltage VADJ to be lower than the lower end voltage TRILOW of the triangular wave signal TRI by at least the amount of the input offset voltage.
In this way, monotonicity of the input/output characteristic of the coil load driving circuit 101 is maintained as shown in FIG. 9(b) by providing an insensitive zone. However, linearity is not maintained in the region of small difference of the input control voltage VIN and input reference voltage VREF. In order to effect improvement in this respect, a coil load driving circuit described below may be considered.
The coil load driving circuit 201, as shown in FIG. 11, includes a voltage-current converter 231 that inputs an input control voltage VIN at its inversion input terminal and inputs an input reference voltage VREF at its non-inversion input terminal, and that output currents of both positive and negative polarities proportional to the difference of the inputted voltages; two bias resistors 232, 233 having first ends connected with the respective outputs of the voltage-current converter 231 and second ends connected with the center voltage VCEN of the triangular wave signal TRI that is output by an oscillator (OSC) 213; a first PWM comparator 214 that inputs the voltage that is generated by passage of the output current of positive polarity of the voltage-current converter 231 to the bias resistor 232 (first transfer voltage VTR1) at its inversion input terminal and that inputs the triangular wave signal TRI at its non-inversion input terminal, and that controls a first output buffer 211, to be described, by outputting a first PWM signal PW1 obtained by comparing these inputted voltage and signal; a second PWM comparator 215 that inputs a voltage generated by passage of output voltage of negative polarity of the voltage-current converter 231 to the bias resistor 233 (second transfer voltage VTR2) at its inversion input terminal and that inputs the triangular wave signal TRI at its non-inversion input terminal, and that controls a second output buffer 212, to be described, by outputting a second PWM signal PW2 obtained by comparing these inputted voltage and signal; a first output buffer 211 that outputs a PWM pulse at one terminal of the motor 5, being connected with the downstream end of the first PWM comparator 214; and a second output buffer 212 that outputs a PWM pulse at the other terminal of the motor 5, being connected with the downstream end of the second PWM comparator 215.
The operation of this coil load driving circuit 201 will now be described with reference to the waveform diagram of FIG. 12. FIG. 12 shows the waveform generated at the various sections when the input control voltage VIN is increased linearly. Referring to FIG. 12, (a) shows the input control voltage VIN, (b) shows the first and second transfer voltages VTR1, VTR2 and the triangular wave signal TRI, (c) shows the PWM pulse at the first output terminal OUT1 (i.e. the first PWM signal PW1), (d) shows the PWM pulse at the second output terminal OUT2 (i.e. the second PWM signal PW2). As can be seen from this waveform diagram, when the input control voltage VIN is lower than the input reference voltage VREF and the difference is large, the first transfer voltage VTR1 is high and the pulse width of the first PWM signal PW1 (i.e., the high-level period thereof) is small. In contrast, the second transfer voltage VTR2 is low and the pulse width of the second PWM signal PW2 (i.e., the high-level period thereof) is large. When the input control voltage VIN rises, the first transfer voltage VTR1 falls, causing the pulse width of the first PWM signal PW1 to gradually become larger, and the second transfer voltage VTR2 rises, causing the pulse width of the second PWM signal PW2 to become gradually smaller. The first and second PWM signals PW1, PW2 are output as PWM pulses for PWM driving of the motor 5 from the first and second output buffers 211, 212, respectively. Consequently, when the input control voltage VIN is lower than the input reference voltage VREF, the pulse width of the second PWM signal PW2 (i.e., the high-level period thereof) is larger than the first PWM signal PW1, so a period is produced in which negative voltage is applied between the two terminals of the motor 5, causing the motor 5 to rotate in the reverse direction. As the input control voltage VIN rises, the period in which negative voltage is applied becomes shorter, so the torque of the motor 5 is decreased. When the input control voltage VIN is higher than the input reference voltage VREF, since the pulse width of the first PWM signal PW1 is larger than that of the second PWM signal PW2, a period is produced in which positive voltage is applied between the two terminals of the motor 5, so the motor 5 performs rotation in the forward direction. As the input control voltage VIN rises, this period in which positive voltage is applied becomes longer, so the torque of the motor 5 becomes larger.
In this way, in this coil load driving circuit 201, a first PWM signal PW1 corresponding to the first transfer voltage VTR1 that is increased and decreased maintaining monotonicity and linearity, and a second PWM signal PW2 corresponding to the second transfer voltage VTR2 that is increased and decreased maintaining monotonicity and linearity are output from the first and second output buffers 211, 212 as PWM pulses to drive the motor 5, in accordance with the difference between the input control voltage VIN and input reference voltage VREF. In this coil load driving circuit 201, since evaluation of polarity is not used, as it is in the case of the polarity comparator 115 of the coil load driving circuit 101, there is no possibility of monotonicity or linearity being lost in the vicinity of equality of the input control voltage VIN and the input reference voltage VREF, so an input/output characteristic as shown by the characteristic diagram of FIGS. 13(a) and 13(b) can be obtained.
However, typically, considerable radiation noise is produced due to the switching in apparatuses using PWM pulses, so the effects of crosstalk, etc., on other signals becomes a problem. Consequently, counter-measures need to be adopted to reduce the radiation noise as much as possible at the source where this considerable radiation noise is generated. In a coil load driving circuit, the current output capability of an output buffer that outputs a PWM pulse driving a coil load such as a motor is large, so this represents a major source of generation of radiation noise.
In the case of the coil load driving circuit 201 that is devised aiming to maintain linearity in the region of a small difference of the input control voltage VIN and input reference voltage VREF as described above, an input/output characteristic maintaining monotonicity and linearity can indeed be obtained, but, because of the continual switching of the output buffers that output the PWM pulses to the two terminals of the motor, more radiation noise is produced than in the case of the coil load driving circuit 101. In particular, when the motor is stationary, the coil load driving circuit 101 does not output a PWM pulse, but, in the case of the coil load driving circuit 201, PWM pulses of 50% duty are output to the two terminals of the motor. When used, for example, for focus adjustment or tracking adjustment of an optical disc device, the ordinary condition of the motor is that the motor is stationary and so it is undesirable that radiation noise should be generated by continual switching of the output buffers even in this case.
An example of an optical disc device is shown in FIG. 14. In this optical disc device, a focus adjustment coil load driving circuit 511 and a tracking adjustment coil load driving circuit 512 that are included in a servo circuit 501 drive a focus adjustment coil load 513 and a tracking adjustment coil load 514 that are included in the optical pickup 502.